Embodiments of the present disclosure are related to power conversion systems, and more particularly to multi-level power conversion systems.
There is a growing need to deliver power over long distances to remote locations. Alternating current (AC) transmission systems are considered inefficient for transmitting high voltage power over long distances as the reactance of an AC transmission cable reduces the load carrying capability of the AC cable. High voltage direct current (HVDC) transmission provides a more efficient way to transmit high voltage power over long distances.
Power converters are often used to convert AC power to direct current (DC) power at a transmitting substation and to convert the transmitted DC power back to AC power at a receiving substation in HVDC transmissions. One or more DC fault conditions often occur during operation of the power converters. For example, the DC fault condition may include a DC short circuit. The DC fault conditions induce transient current overshoots on a DC side of the power converter, which in turn may result in a current overshoot on an AC side of the power converter. Components with larger power ratings have been employed in the currently available power converters to overcome such DC faults. However, these components add undesirable costs to the HVDC transmission systems.
Furthermore, in an event of the DC fault, the power converters are configured to shut down and restart their operations once the DC fault is cleared in the HVDC transmission system. Such a temporary suspension of operation leads to a cascading effect in the HVDC transmission system and may result in an HVDC transmission system collapse.
Various approaches have been employed to provide DC fault ride-through capability to the power converters. The DC fault ride-through capability enables the power converters to overcome the DC fault without having to shut down the power converters. Unfortunately, use of the aforementioned approaches call for the use of an increased number of components, higher costs, higher complexity and lower efficiency.